A wireless communication system comprises a number of base stations, distributed over a geographic area, to provide service for a number of wireless devices, such as personal digital assistants (PDA), cellular phones, portable computers and other suitable devices that can move throughout the geographic area. A problem with the wireless device exists when there are regions of interference or coverage holes in the service area, resulting in the loss of communication on the forward or reverse channel. A coverage hole is an area within the serving area of the service provider in which a wireless device cannot receive acceptable service from the serving base station. For example, there may be a geographic area for which, because of terrain and building clutter, weak service or no service at all is available (e.g., a geographic coverage hole). Additionally, regions of interference within the coverage area may exist in areas where one or more transmitters interfere with the transmit signal of another wireless device. For example, in densely populated urban areas, wireless devices may transmit excessive interference on the reverse channel. When the wireless device moves through a coverage hole or an interference region, it is often the case that the wireless device will emerge from the coverage hole within a short period of time.
Another hindrance to wireless service is known as the unbalanced link problem. In the art, a propagation path loss on the forward channel (base station to wireless device) is generally assumed to track with the propagation path loss on the reverse link (wireless device to base station). However, due to, for example, varying levels of interference on the forward and reverse channel and the complexities of network planning, the forward and reverse channels may be somewhat imbalanced at times. Occasionally the forward channel is impaired more then the reverse channel. This condition is generally referred to as being “forward channel limited.”
The forward channel may be strong enough that it is receivable by the wireless device in some localized areas, whereas the base station is not able to receive the reverse channel messages transmitted by the wireless device. This latter condition is generally referred to as being “reverse link limited.” A reverse link limited condition might also be caused by an interference region, a coverage hole, or perhaps because the service provider has increased the effective radiated power (ERP) of the base station in order to overcome interference by other nearby base stations belonging to a competing system. Nevertheless, the forward channel may still be strong enough in this case to be successfully demodulated by the wireless device. In this reverse link limited situation, the wireless device would be unable to acknowledge a page, unable to originate a call, or even register with the CDMA base station, even though a satisfactory service indication is displayed.
According to one method, upon a failure of communication with a current communication system, the wireless device attempts to acquire one of the less preferred communication systems. The wireless device attempts acquisition according to a list of preferred communication systems in a preferred roaming list (PRL), upon a communication failure with the current communication system.
According to another method, upon a failure of communication with a current communication system, the wireless device attempts to re-register with the current communication system. However, if the network fails to acknowledge the registration attempts, the wireless device will continue to attempt to register on the reverse link limited channel. As a result, the wireless device may again experience a lack of service each time the wireless device travels through the area experiencing the reverse link limited channel.